CHAPTER 3
How the Body Gets Energy While Fasting Any day of the year, whether you’re fasting or feasting, the body gets energy from either glucose or fat. Amino acids from proteins can also provide energy, but since protein is so vital to body functions, there are many mechanisms in place to spare amino acids from being used for energy (one of which is the production of ketones, as you shall see in this chapter). In a healthy person who eats a typical diet of carbs, fats, and proteins, the body will primarily use glucose for energy during and after meals, and primarily fat when it has been a long time since a meal. A metabolically healthy person will swing back and forth between functioning on a glucose-based metabolism and a fat-based one, a lot like a hybrid vehicle that can be alternately powered by electricity or gasoline, depending on its mode of operation. The main signal that tells the body which fuel source to use is insulin. In a healthy person, when insulin levels are high, the main fuel source will be glucose. When insulin levels are low, fat will take over as the main energy source. If you follow a ketogenic diet, then you should have persistently low insulin levels and your “car” will mostly run on fat, without switching over to the glucose fuel. But remember: that fat can come from either foods you eat or the fat you carry on your body, depending on how much you’re eating. So if it’s weight loss that you’re after, don’t add too much new fat to the existing supply! Spend some time fasting, even if you are following a ketogenic diet. Since the human body needs energy all the time, regardless of whether food is available, it developed very good mechanisms for storing energy after food is taken in and later drawing on those storage depots when food is not available. For most of human history, food was much more scarce than it is today, and the body needed to use these stores a lot. Because of this ability to store food energy, we don’t have to eat all the time. There’s no physical need for a regular daily meal schedule. The default pattern of breakfast, lunch, and dinner is just something we made up! You have no obligation to follow it.
Energy Storage The body has specific storage depots for both glucose and fat. Glucose is only stored for short-term use, about a day or so. Fat storage, on the other hand, is apparently limitless. An average person can go some weeks without eating, relying exclusively on their body fat for energy. Someone with more fat can, as a general rule, fast longer than a thin person if they have the water, vitamins, and minerals they need.
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